This work provides insight into carrier dynamics in a model photoelectrochemical system comprised of a semiconductor, metal oxide, and metal. To isolate carrier dynamics from catalysis, a common catalytic metal (Pt) is used in concert with an outer-sphere redox couple. Silicon (111) substrates were surface-functionalized with electronegative aryl moieties (p-nitrophenyl and m-dinitrophenyl). A mixed monolayer using p-nitrophenyl/methyl exhibited high surface quality as determined by X-ray photoelectron spectroscopy (low surface SiO x content) and low surface recombination velocity. This substrate also exhibited the expected positive surface dipole, as evidenced by rectifying J–V behavior on p-type substrates, and by positive photovoltage measured by surface photovoltage spectroscopy. Its close molecular relative m-dinitrophenyl exhibited poor electronic surface quality as indicated by high SiO x coverage and high surface recombination velocities (S > 3000 cm s–1). Photoelectrochemical J–V measurements of p-type Si-functionalized surfaces in contact with a high concentration (50 mM) of methyl viologen (MV2+) in aqueous media revealed V OC values that are correlated with the measured barrier heights. In contrast, low-concentration (1.5 mM) MV2+ experiments revealed significant contributions from surface recombination. Next, the electronic and (photo)­electrochemical properties were studied as a function of ALD metal oxide deposition (TiO2, Al2O3) and Pt deposition. For the m-dinitrophenyl substrate, ALD deposition of both TiO2 and Al2O3 (150 °C, amorphous) decreased the surface recombination velocity. Surprisingly, this TiO2 deposition resulted in negative shifts in V OC for all surfaces (possibly ALD-TiO2 defect band effects). However, Pt deposition recovered the efficiencies beyond those lost in TiO2 deposition, affording the most positive V OC values for each substrate. Overall, this work demonstrates that (1) when carrier collection is kinetically fast, p-Si(111)–R devices are limited by thermal emission of carriers over the barrier, rather than by surface recombination. And (2) although TiO2|Pt improves the PEC performance of all substrates, the beneficial effects of the underlying (positive) surface dipole are still realized. Lastly (3) Pt deposition is demonstrated to provide beneficial charge separation effects beyond enhancing catalytic rates.

Photoelectrochemical (PEC) device efficiency depends heavily on the energetics and band alignment of the semiconductor|overlayer junction. Exerting energetic control over these junctions via molecular functionalization is an extremely attractive strategy. Herein we report a study of the structure− function relationship between chemically functionalized pSi(111) and the resulting solar fuels performance. Specifically, we highlight the interplay of chemical structure and electronic coupling between the attached molecule and the underlying semiconductor. Covalent attachment of aryl surface modifiers (phenyl, Ph; nitrophenyl, PhNO 2 ; anthracene, Anth; and nitroanthracene, AnthNO 2 ) resulted in high-fidelity surfaces with low defect densities (S < 50 cm/s). Electrochemical characterization of these surfaces in contact with methyl viologen resulted in systematically shifted band edges (up to 0.99 V barrier height) and correspondingly high photoelectrochemical performance (V oc up to 0.43 V vs MV 2+ ) consistent with the introduction of a positive interfacial dipole. We extend this functionalization to HER conditions and demonstrate systematic tuning of the HER V oc using pSi(111)-R|TiO 2 |Pt architecture. Correlation of the shifts in barrier height with the photovoltage provides evidence for nonideality despite low surface recombination. Critically, DFT calculations of the electronic structure of the organic-functionalized interfaces show that the molecule-based electronic states effectively hybridized with the silicon band edges. A comparison of these interfacial states with their isolated molecular analogues further confirms electronic coupling between the attached molecule and the underlying semiconductor, providing an induced density of interfacial states (IDIS) which decreases the potential drop across the semiconductor. These results demonstrate the delicate interplay between interfacial chemical structure, interfacial dipole, and electronic structure.

Bioelectrocatalytic synthesis is the conversion of electrical energy into value‐added products using biocatalysts. These methods merge the specificity and selectivity of biocatalysis and energy‐related electrocatalysis to address challenges in the sustainable synthesis of pharmaceuticals, commodity chemicals, fuels, feedstocks and fertilizers. However, the specialized experimental setups and domain knowledge for bioelectrocatalysis pose a significant barrier to adoption. This review introduces key concepts of bioelectrosynthetic systems. We provide a tutorial on the methods of biocatalyst utilization, the setup of bioelectrosynthetic cells, and the analytical methods for assessing bioelectrocatalysts. Key applications of bioelectrosynthesis in ammonia production and small‐molecule synthesis are outlined for both enzymatic and microbial systems. This review serves as a necessary introduction and resource for the non‐specialist interested in bioelectrosynthetic research.

Metal hydride species have proven to be a crucial chemical motif across chemical disciplines. As the key intermediate in electrocatalytic hydrogen evolution by molecular catalysts, understanding how metal hydrides are formed and how they react has allowed the design of more efficient electrocatalysts. Independently to the field electrocatalysis, metal hydride species have been critical for organometallic catalysis as reagents for hydrogen atom transfer (HAT), in particular the activation of alkenes to a diverse array of hydrofunctionalization products. Recently, these two fields have been explicitly linked via the emergence of electrocatalytic hydrogen atom transfer (e-HAT); utilizing primarily Cobalt based catalysts (salen and bipyridine) members of the NSF Center for Synthetic Organic Electrochemistry (CSOE) have shown that in situ electro-generated cobalt hydride species can catalyze highly sought-after organic transformations of alkenes, such as alkene isomerizations and enantioselective hydrocyanation reactions. However, from an electrochemical perspective these reactions remain poorly understood, severely limiting the design of new hydrofunctionalization reactions. Here, cyclic voltametric studies of Co(salen) provide a sorely needed mechanistic framework to understand these multi-step electrocatalytic reactions. Using model homolytic reactions we establish rate constants for cobalt hydride formation as well as a relationship between hydride donor ability and alkene activation. Hammett analysis of a series of modified salen ligands shows changes in the rate determing step, suggesting tunability in future organic electrosynthetic reactions. Finally, we detect key off-cycle intermediates that inhibit catalytic turnover and suggest further optimization in yield and enantioselectivity selectivity are possible. In summary, we contend that these mechanistic studies provide an important template for studying complex, multistep organic reactions from the perspective of traditional molecular electrocatalysis as developed by Jean-Michel Savéant.

In this work, we report the effects of NNS-thiolate ligands and nuclearity (monomer, dimer) on the stability of iron complexes related to the active site of monoiron hydrogenase (Hmd). A thermally stable iron(II) dicarbonyl motif is the core feature of the active site, but the coordination features that lead to this property have not been independently evaluated for their contributions to the {Fe(CO)} stability. As such, non-bulky and bulky benzothiazoline ligands (thiolate precursors) were synthesized and their iron(II) complexes characterized. The use of non-bulky thiolate ligands and low-temperature crystallizations result in isolation of the dimeric species [(NNS)Fe(CO)(I)] (1), [(NNS)Fe(CO)(I)] (2), and [(NNS)Fe(CO)(I)] (3), which exhibit dimerization via thiolato (μ-S) bridges. In one particular case (unsubstituted NNS ligand), the pathway of decarbonylation and oxidation from 1 was crystallographically elucidated, via isolation of the half-bis-ligated monocarbonyl dimer [(NNS)Fe(CO)]I (4) and the fully decarbonylated and oxidized mononuclear [(NNS)Fe]I (5). The transformations of dicarbonyl complexes (1, 2, and 3) to monocarbonyl complexes (4, 6, and 7) were monitored by UV/vis, demonstrating that 1 and 3 exhibit longer t (80 and 75 min, respectively) than 2 (30 min), which is attributed to distortion of the ligand backbone. Density functional theory calculations of isolated complexes and putative intermediates were used to corroborate the experimentally observed IR spectra. Finally, dimerization was prevented using a bulky ligand featuring a 2,6-dimethylphenyl substituent, which affords mononuclear iron dicarbonyl complex, [(NNS)Fe(CO)Br] (8), identified by IR and NMR spectroscopies. The dicarbonyl complex decomposes to the decarbonylated [(NNS)Fe] (9) within minutes at room temperature. Overall, the work herein demonstrates that the thiolate moiety does not impart thermal stability to the {Fe(CO)} unit formed in the active site, further indicating the importance of the organometallic Fe-C(acyl) bond in the enzyme.